This application claims the priority of German patent document 100 34 399.6, filed Jul. 14, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a fuel cell system in which water contained in the exhaust-air stream is recovered, and to a method of operating such a fuel cell system.
Fuel cell systems are known for example for the operation of an electric motor. A distinction is made between fuel cells to which a fuel is supplied directly and those to which hydrogen obtained by reforming of hydrocarbons, such as gas, diesel, naphtha or alcohols such as methanol is supplied as the fuel. In addition to fuel, a fuel cell also requires oxygen, which is usually supplied by the ambient air. The fuel cell produces water and electrical energy by electrochemical conversion from hydrogen supplied on the anode side and oxygen supplied on the cathode side. Water is often also added as a coolant and/or as a humidifying agent to the streams of substance on the anode side and/or cathode side. The exhaust-air stream of a fuel cell system therefore contains water, including water vapor. Proposals exist for mechanically and/or thermally separating this water out of the exhaust-air stream, in order to recover it for the process. However, components suitable for this purpose are relatively large, or require additional energy, thereby reducing the efficiency of the system.
Apart from air, water is also required for the reforming of hydrocarbons. Known hydrocarbon reformers are schematically represented in FIGS. 4 and 5. FIG. 4 shows the common structure of a hydrocarbon reformer and of a fuel cell system, while FIG. 5 shows a structure modified according to International Patent Document WO 98/08771. Corresponding components are denoted by the same reference numerals in FIGS. 4 and 5.
In principle, as illustrated in FIG. 4, hydrocarbon, air and water are supplied to the reactor 1 via the lines 2, 4 and 3, respectively, from which carbon dioxide and hydrogen can be produced in a chemical reaction. Depending on the mixing ratio, also produced are carbon monoxide components, which are oxidized in a selective oxidation stage 9 to form carbon dioxide. The energy released in this process can be supplied to the reforming process in the form of heat. Carbon monoxide produced can also be converted by a water-gas shift reaction in a high-temperature stage 6 and a low-temperature stage 8, using water to form carbon dioxide and hydrogen.
Furthermore, there are concepts which provide rapid cooling of the reformate gas downstream of the reforming reactor 1 by the addition of water (quenching). Heat exchangers (WÜ) are identified in the figures by the reference numeral 7. Part of the water produced may be separated off before the hydrogen produced in the reforming stage is fed to the fuel cell 11. A fan 10 may be used for cooling the components.
The fuel cell system 11 is subsequently supplied on the anode side with hydrogen and on the cathode side with atmospheric oxygen via the compressor 14. Water and electrical energy are produced from these by electrochemical conversion. Water can be separated off from the exhaust gas of the fuel cell via a heat exchanger 7 and unburned hydrogen can be converted into water via a catalytic burner 12, the heat produced in the process being supplied to a heat exchanger 7. The residual exhaust gas remaining is transported to the outside via an expander 13.
FIG. 5 shows a slightly modified structure of a hydrocarbon reformer. (Apart from the following differences, the process sequence corresponds here to the sequence shown in FIG. 4.) The thermal energy given off by the catalytic burner 12 is used in the heat exchanger 7 (WÜ 5) for the vaporization of a liquid hydrocarbon such as gasoline, which is supplied to the system via the line 2. Water is fed to the system via a heat exchanger 7 (WÜ 2) and enters the high-temperature unit 6 as water vapor for the water-gas shift reaction. Water vapor leaving this unit 6 is supplied together with the vaporized gasoline to the reforming reactor 1 via the line 3. Air passes via the heat exchanger 7 (WÜ 1) via the line 4 to the reforming reactor 1. Furthermore, liquid water is required for cooling of the product in the quenching stage 5.
In the fuel cell systems described, with upstream hydrocarbon reforming, all of the water must be recovered completely within the system, since otherwise the missing water would have to be provided by filling up at a station in the case of a mobile application of the system. In the case of stationary use, there would be increased water consumption of the system, which cannot be tolerated for reasons of environmental protection and cost-effectiveness.
U.S. Pat. No. 6,013,385 discloses water recovery from the cathode exhaust gas of a fuel cell, water recovery by means of a rotating adsorption system or enthalpy wheel being used in addition to the separation of water. A porous material, such as a molecular sieve or zeolite, through which moist, cold exhaust cathode gas flows on one side and hot air, which has to be additionally humidified before entry into the cathode side, flows on the other side, mounted on a shaft is used as the enthalpy wheel. A continuous adsorption and desorption of water is achieved by turning the adsorption material (enthalpy wheel).
Other concepts provide the recovery of the process water by means of condensation downstream of the fuel cell. For this purpose, very low temperatures and/or a high system pressure are required in the condenser. The following difficulties occur in this case. Condensation temperatures below 55° C. can only be realized with difficulty in the case of mobile applications, since the vehicles have to be able to operate even at outside temperatures of over 40° C. and the required vehicle radiators must in this case be of a very sophisticated design (increase in overall size). On the other hand, the required condensation temperature can be increased by increasing the system pressure. However, compressors are necessary to apply the required system pressure, which again uses part of the electrical energy produced in the fuel cell. Also, this part of the energy can only be partially recovered by an expansion engine, so that there is an overall reduction in the system efficiency. Finally, in particular for the startup of the system, the process water must be stored in a tank, creating problems regarding frost resistance.
One object of the present invention is to solve the problems described above in the prior art.
Another object of the invention is to provide a fuel cell system, in particular with upstream reforming for hydrocarbons and/or alcohols, which can be operated at low pressure (atmospheric pressure) and which dispenses as completely as possible with an external supply of water.
This and other objects and advantages are achieved by the fuel cell method and apparatus according to the invention in which the water contained in the moist exhaust air of the fuel cell system is removed from the exhaust air stream by means of absorption and is released again by subsequent desorption. The released water can be returned fully or partially to the process. For this purpose, an absorption unit and a desorption unit are provided according to the invention, the absorption unit being connected into the exhaust-gas line of the fuel cell system, so that the water contained in the exhaust gas can be taken up by the absorbing agent of the absorption unit. A desorption unit, which contains the absorbing agent charged with water, is connected to the fuel cell system for the return of the desorbed water.
Water recovery by means of absorption/desorption according to the invention allows virtually complete recovery of the water used in the process, without requiring an increase in the system pressure. This makes it possible to dispense virtually completely with an external water supply, which is a necessity for use of the fuel cell systems in mobile systems. Furthermore, it is possible to dispense with a compressor, increasing the compactness of the system, and simultaneous reducing costs. Finally, if a suitable absorbing agent is chosen, complete recovery of the absorbed water can be achieved in the following desorption process.
Glycol has proven to be a particularly advantageous absorbing agent, with ethylene glycol, diethylene glycol or triethylene glycol representing suitable absorbing agents in particular. A mixture of one or more of these with water in a specific weight ratio proves to be particularly suitable for physically absorbing water. On account of the great difference between the boiling points of glycol and water, water can be easily separated out of such glycol solutions charged with water. A weight ratio which lies between 0-60% by weight of glycol and 0-40% by weight of water is favorable.
For the desorption, air sucked in from the surroundings, which can be heated if need be, is suitably used. Increasing the solution temperature for the desorption of the water is also possible. However, the glycol solution may also or alternatively be heated.
It is advantageous to supply the recovered water to a reformer for hydrocarbons and/or alcohols, which produces the fuel for the fuel cell. For this purpose, the desorption unit may be supplied with warm, dry air, which leaves the desorption unit after giving off part of its heat and taking up the water contained therein. The warm and moist air is then supplied to the reforming reactor. The reforming reaction may in this case take place with air or water, or with an air/water mixture.
The absorption and desorption units are advantageously combined in such a way that the absorbing agent is circulated, that is to say the absorbing agent leaves the desorber in the regenerated state and re-enters the absorber. After reaching a certain degree of saturation, the absorbing agent is fed back into the desorber.
This process may take place continuously or periodically, in one or more stages, and must be optimized with respect to the requirements during use.
Use of a centrifugal reactor, in which very high mass transfer coefficients can be achieved, proves to be particularly suitable. As a result, the size of the absorption/desorption unit can be reduced. Such centrifugal reactors are described in the patents U.S. Pat. Nos. 4,283,255 and 4,400,275. They can be used generally for mass transfer between two fluids. For this purpose, a liquid phase is introduced into the centrifugal reactor close to the axis of rotation, the centrifugal forces occurring causing the liquid to penetrate a space filled with a permeable packing and rotating about the axis of rotation. A gaseous phase is introduced into the outer space surrounding the rotating space, penetrates the liquid in countercurrent and leaves the reactor by being discharged close to the axis of rotation.
In the present invention, such a centrifugal reactor can be used both for the absorption and for the desorption. In absorption, the absorbing agent (glycol) is supplied to the centrifugal reactor in the liquid phase and, in countercurrent to this, the moist fuel cell exhaust gas is supplied to it in the gaseous phase. In desorption, the absorbing agent charged with water is supplied to the centrifugal reactor as a liquid, and a heated air stream is supplied to it as gas. It is also possible to use a flash chamber, a cyclone or the like.